C9orf72 and fragile X syndrome: However, similar repeat expansion has been proven to be the cause of many other neurodegenerative diseases such as fragile X syndrome (FXS) (15), several types of spinocerebellar ataxia (SCA31 and SCA37) (16, 17), C9orf72-related disorder (18), and, most recently, neuronal intranuclear inclusion disease (NIID) (19).